Receptor tyrosine kinases (RTKs) are cell surface proteins that transmit signals from the extracellular environment to the cell cytoplasm and nucleus to regulate cellular events such as survival, growth, proliferation, differentiation, adhesion and migration.
The TAM subfamily consists of three RTKs including Tyro3, AXL and Mer (Graham et al., 2014, Nature Reviews Cancer 14, 769-785; Linger et al., 2008, Advances in Cancer Research 100, 35-83). TAM kinases are characterized by an extracellular ligand binding domain consisting of two immunoglobulin-like domains and two fibronectin type III domains. Two ligands, growth arrest specific 6 (GAS6) and protein S (PROS1), have been identified for TAM kinases. GAS6 can bind to and activate all three TAM kinases, while PROS1 is a ligand for Mer and Tyro3 (Graham et al., 2014, Nature Reviews Cancer 14, 769-785).
AXL (also known as UFO, ARK, JTK11 and TYRO7) was originally identified as a transforming gene from DNA of patients with chronic myelogenous leukemia (O'Bryan et al., 1991, Mol Cell Biol 11, 5016-5031; Graham et al., 2014, Nature Reviews Cancer 14, 769-785; Linger et al., 2008, Advances in Cancer Research 100, 35-83). GAS6 binds to AXL and induces subsequent auto-phosphorylation and activation of AXL tyrosine kinase. AXL activates several downstream signaling pathways including PI3K-Akt, Raf-MAPK, PLC-PKC (Feneyrolles et al., 2014, Molecular Cancer Therapeutics 13, 2141-2148; Linger et al., 2008, Advances in Cancer Research 100, 35-83).
MER (also known as MERTK, EYK, RYK, RP38, NYK and TYRO12) was originally identified as a phospho-protein from a lymphoblastoid expression library (Graham et al., 1995, Oncogene 10, 2349-2359; Graham et al., 2014, Nature Reviews Cancer 14, 769-785; Linger et al., 2008, Advances in Cancer Research 100, 35-83). Both GAS6 and PROS1 can bind to Mer and induce the phosphorylation and activation of Mer kinase (Lew et al., 2014). Like AXL, MER activation also conveys downstream signaling pathways including PI3K-Akt and Raf-MAPK (Linger et al., 2008, Advances in Cancer Research 100, 35-83).
TYRO3 (also known as DTK, SKY, RSE, BRT, TIF, ETK2) was originally identified through a PCR-based cloning study (Lai et al., Neuron 6, 691-70, 1991; Graham et al., 2014, Nature Reviews Cancer 14, 769-785; Linger et al., 2008, Advances in Cancer Research 100, 35-83). Both ligands, GAS6 and PROS1, can bind to and activate TYRO3. Although the signaling pathways downstream of TYRO3 activation are the least studied among TAM RTKs, it appears that both PI3K-Akt and Raf-MAPK pathways are involved (Linger et al., 2008, Advances in Cancer Research 100, 35-83). AXL, MER and TYRO3 are found to be over-expressed in cancer cells.
Accordingly, there is a need for compounds and methods of use thereof for the modulation of TAM kinases in the treatment of cancer.